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An Austrian startup’s low-cost gadget which lets people in developing countries know when water is safe to drink has been approved by the World Health Organization (WHO).

A cheap and effective way to turn contaminated water into clean, drinkable water is to fill clear plastic PET bottles with the source water and expose them to direct sunlight until the UV rays disinfect the water.

But it's not easy to determine exactly when the water is disinfected, which means that people may end up drinking water that isn't fully disinfected.

However, Wadi is a solar-powered portable gadget developed by the Vienna-based Helioz startup, which measures the UV radiation hitting the bottle and determines exactly how long the water needs to be in the sun and lets users know when it’s ready to drink.

A smiley face on the gadget alerts the user as soon as the water has reached a bacterially and virally safe level.

The device can be used and shared by several people, as many bottles of water can be purified simultaneously.

Wadi meets WHO microbiological performance criteria for household water treatment technologies and is classified as providing targeted protection.

Helioz founder Martin Wesian caught cholera when travelling in Venezuela after drinking contaminated water, and since then was inspired to find innovative solutions to reducing waterborne diseases.

The Wadi device is made in Upper Austria and sells in Europe for €30. If NGOs order in bulk, the price for each unit is lowered to around €10 or €15. Around 10,000 Wadi devices are currently in use, mainly in the Philippines, Kenya, Uganda, Ethiopia, Ghana, Mali and India.

Wesian says that the advantage of Wadi over other water purifying technologies is that it does not need batteries and will last for years.

Worldwide, there are around 660 million people without access to clean drinking water and every 90 seconds a child under the age of five dies as a result of diseases such as cholera and typhoid.